Note that the ETX680 and ETX900 share the same case size: "E". Either will fit in the same battery tray. If you have an electrically dependent engine and a single battery, you might consider using the 15.6Ah ETX900 instead of the 12.4Ah ETX680. Doing so will achieve longer alternator-out endurance, or the same endurance with less load shedding required. Weight difference is 0.8 lb.
Even without an electrically dependent engine, there are advantages to using the ETX900. It can supply higher starting current and tolerate higher recharge current. Those things, combined with its higher initial capacity mean it will likely last longer in the same environment than an ETX680.
Whether the longer lifespan will favorably amortize the $70 higher price is an open question...
Edited to add:
If you have a 912iS (and presumably a 915iS), neither of these batteries meet the minimum battery ampere-hour capacity listed in the Rotax installation manual, but the ETX900 is nearly big enough.
Originally Posted by Rotax 912iS Installation Manual